I think your idea to compress the mp3s on the PC and copy them back is the right one (regardless of excluding them from the backup).
Have you tried a dedicated copy program to copy the folders to the PC (something like teracopy for instance)? I also can recommend the program Beyond Compare (which isn't free but well worth the money) which not only is an excellent comparison and checking tool but also very reliable syncs and copies folders and files.
Have you tried a dedicated copy program to copy the folders to the PC (something like teracopy for instance)? I also can recommend the program Beyond Compare (which isn't free but well worth the money) which not only is an excellent comparison and checking tool but also very reliable syncs and copies folders and files.